Stamens 5 or rarely 4, inserted in the tube; filaments free or rarely united; anthers often sagittate, free or connivent around the stigma, rarely adherent to the latter, 2-celled, opening lengthwise, connective often produced at the apex; pollen granular; disk usually present, annular, cupular or of separate glands

Gynoecium

Style 1, split at the base or entire, thickened and stigmatose below the apex; ovules 2 or more in each carpel
Ovary superior, 1-celled with 2 parietal placentas or 2-celled with the placentas adnate to the septa, or carpels 2 and free or connate only at the base with ventral placentas in each carpel

Fruits

Fruit entire and indehiscent or of 2 separate carpels, baccate, drupaceous or follicular

Seeds

Seeds mostly with endosperm and large straight embryo, often winged or appendaged with long silky hairs

Calyx often with colleters inside, free or united at the base, imbricate in bud

Corolla

Corolla tubular, contorted or occasionally valvate, free, overlapping to the right or left in bud, sometimes with a corona

Androecium

Stamens included or exserted, epipetalous, free or connate to the style; filaments often very short, epipetalous; anthers frequently triangular, of two cells longitudinally dehiscent, often partly sterile, sometimes with apical appendages

Gynoecium

Ovary superior or sometimes partly inferior, 1-celled and with 2 parietal placentas, 2-celled and with an axile placenta in each cell or composed of 2, or more, separate or at the base partly united carpels each with an adaxial placenta; ovules 2 to many; style one, often split at the base when carpels more or less separate; pistil head composed of a large variously shaped stigmatic basal part and a stigmoid apex; disk annular, cupular, composed of separate glands, or absent

Fruits

Fruit entire or consisting of two (rarely more) separate or partly united carpels, baccate, drupaceous, or follicular

Seeds

Seeds in dry fruits often winged or with a coma, mostly with an endosperm and a large embryo

Note

The corona presents many diagnostic characters for generic and specific recognition. Following the system devised by Liede & Kunze in Pl. Syst. Evol. 185: 275–284 (1993), it may be corolline (derived from the corolla) or gynostegial (from the staminal column). Corolline coronas can be divided into those occuring in the corolla lobe sinuses and those forming an annulus in the corolla tube. Gynostegial coronas again have two basic elements, a staminal corona attached dorsally to the stamens, and an interstaminal corona, and these elements can be combined in a number of ways: e.g. staminal corona lobes only; a fused ring of staminal and interstaminal lobes; fused staminal and interstaminal corona with additional staminal lobes.
Notes on floral structures within the more derived subfamilies of Apocynaceae: the five stamens are fused apically to the expanded stylar head and together form a compound structure called the gynostegium. The ovary is therefore almost entirely concealed within a ring of stamens (the staminal column) whose filaments are usually fused into a tube, but free in subfamily Periplocoideae. Only the sterile apex of the stylar head remains visible, level with or extending beyond the anthers; the receptive parts of the stylar head are on its underside behind the frequently sclerified margins of the anthers which form a chamber for the deposition of pollen. The pollen transfer apparatus is formed by secretions from the stylar head and, in periplocoid genera, generally consists of a spatulate translator onto which pollen tetrads, or more rarely pollen masses (pollinia), from adjacent anthers are shed; in the two remaining subfamilies, the secretions form a central corpusculum linked by caudicles or translator arms to 2 or 4 pollinia of adjacent anthers – the unit is transferred in its entirity from one flower to another and is called a pollinarium.

Stamens inserted on the corolla; filaments free from each other or exceptionally (not in FZ area) united in a tube, often very short, frequently continued downwards as ridges at the corolla inside; anthers frequently triangular, connivent over and often coherent with the stigma, 2-celled, often partly sterile, sometimes with apical appendages; cells parallel, discrete, dehiscent throughout by a longitudinal slit
Pollen granular

Gynoecium

Ovary superior or sometimes partly inferior, 1-celled and with 2 parietal placentas, 2-celled and with an axile placenta in each cell, or composed of 2, rarely more, separate or at the base partly united carpels each with an adaxial placenta; ovules 2 to many; style one, often split at the base when carpels more or less separate; stigma composed of a large variously shaped part, usually called the clavuncula, with laterally and/or basally the receptive zone, which is — if stigma coherent with the anthers — below the level of coherence, and a small apical usually sterile (also in FZ) so-called stigma

Large shrubs (Calotropis, introduced in New World), lianas, vines, herbs or subshrubs, mostly perennial , often sending up annual shoots from a perennial rootstock, occasionally annual , and rarely strongly succulent (introduced in the Americas); latex usually white, but clear in non-native, succulent Ceropegieae. Leaves simple , entire , most commonly opposite but occasionally whorled , or reduced to a scale or spine (in introduced succulent genera). Flowers actinomorphic , bisexual , 5- merous except for the paired carpels; corolla fused at least at the base, varying from rotate or shallowly campanulate to tubular or funnel-shaped, lobes twisted in bud and overlapping to the right, or valvate ; coronal structures often present either on corolla (infrequently) or more commonly on gynostegium (formed from fusion of stamens and stylar head ; stamens inserted at base of corolla , filaments fused into a tube, anthers with specialised marginal wings or guide rails; pollen aggregated into pollinia (one per locule , two per pollinarium), these bound by a waxy outer wall, pollinia attached to hard, brown or black, grooved corpusculum by translator arms, forming a pollinarium extracted from the flower as a unit; ovary superior , apocarpous with two carpels, these fused apically to form a stylar head with stigmatic surfaces on underside (secretions from the stylar head are formed into five discrete clip-like structures (corpuscula) which assist in pollen transport). Fruits paired dehiscent follicles (frequently single by abortion). Seeds generally flattened, ovate , mostly with a narrow marginal rim and a tuft of hairs at one end.

Distribution

Distribution in the Neotropics

Throughout, but most diverse in seasonally dry habitats.

Diagnostic

Distinguishing characters (always present)

Subfamily Asclepiadoideae can be characterised by the presence of a gynostegium in which the expanded stylar head is fused to the androecium by cell fusion, and where the narrow gap between the sclerified wings of adjacent anthers form five grooves.

The form of the pollinarium is also diagnostic for the subfamily, and indeed tribes within it can be determined by close observation of pollinarium structure (see below).

Pollen of one antherlocule is aggregated into a pollinium encased by a waxy wall. This pollen mass is linked to another in an adjacent anther by a pair of translator arms and a single corpusculum. So two pollinia linked by translator arms to a single corpusculum form a pollinarium that can be extracted from the flower as a single unit. The corpusculum is usually black or dark brown, and sits at the top of the groove between adjacent anthers.

Other important characters

Other striking floral characters include a corona of one or more whorls. This can be derived from the corolla, or from the gynostegium, usually the stamens, but is occasionally absent.

Key differences from similar families

Subfamilies Apocynoideae and Rauvolfioideae (Apocynaceae in the narrow sense) lack the pollinarium - pollen is generally distributed as tetrads rather than pollinia. If a corona is present, then it is found only on the corolla. The stylar head and the cone of anthers are positioned part way up the corolla tube rather than at the base, and do not form a compound structure, the gynostegium. Fruit morphology is much more varied, particularly in the basal Rauvolfioideae, where seeds lack a coma of hairs, and the fruit is often an indehiscentberry.

Notable genera and distinguishing features

Asclepias - erect herbs or subshrubs; flowers generally brightly coloured, red, yellow or white, corona of 5 lobes on the back of the stamens, concave with a prominent tooth arising from the cavity of the lobe; follicles always single by abortion.

Cynanchum - twiners; corona tubular, at least at the base, around the gynostegium.

Marsdenia - fairly robust twiners, often with large, thickish leaves, some species associated with inselbergs; corolla usually with well-developed tubular portion enclosing the gynostegium, corona of 5, generally fleshyerect lobes on back of stamens; pollinia erect.

Ditassa - mostly twiners with small flowers; corolla lobes mostly not obviously pubescent, corona of five 'double' lobes - each lobe has a ligule on the inner face.

Minaria - ericoid shubshrubs with flowers similar to Ditassa (recently split from Ditassa).

Funastrum - mostly vigorous, free-flowering twiners often with their feet in seasonally flooded pools; stems semisucculent and photosynthetic (leaves often drop off); umbels of white or cream flowers, corona mosly with a tubular outer ring, and 5 fleshy lobes on the back of the anthers.

Oxypetalum - erect herbs or twiners; flowers moderate in size; mostly with conspicuous stylar head appendages (divided into 2 or more lobes); corona lobes arising from the corolla tube NOT on the back of the anthers; pollinaria large, and often with teeth on the translator arms; distribution centred on Brazil.

Philibertia - Andean version of Oxypetalum but with corona on back of stamens not on the corolla, or absent; stylar head appendages variable or absent.

Useful tips for generic identification

If the plant is a stem-succulent with leaves reduced to scales, you are looking at an introduced Stapelia or allied genus in the Ceropegieae. The pollinaria will be held erect, and have a translucent germination zone.

If the pollinia are held erect, i.e. above the position of the corpusculum, and do not possess a translucent germination zone, then the tribe is Marsdenieae. Marsdenia is the only representative of this tribe in the New World - clue: if the anthers extend significantly beyond the position of the corpusculum, there will be space for the pollinia to be held erect. Marsdenia tends also to have a tubular corolla obscuring the gynostegium, and five erect staminal corona lobes. NB. Barjonia (Asclepiadeae) may have erect pollinia, but will never have a tubular corolla obscuring the gynostegium.

If the pollinia are pendant or horizontal relative to the position of the corpusculum, then the plant is in tribe Asclepiadeae - the vast majority of New-World asclepiads, and all the remaining NW genera.

Asclepias can be recognised easily by the combination of erect herb or subshrub AND a staminal corona formed of five concave fleshy organs with a tooth arising from the cavity of each lobe. Corolla and corona are often brightly coloured.

Now look at the pentagonal drum of stamens - the 'head' of the gynostegium. If this is flattened and subdiscoid, with very short anther wings relative to the diameter of the disc, the pollinia will be displaced laterally in relation to the corpusculum, and you are looking at Gonolobinae - the two big genera being Matelea and Gonolobus (see below for diagnostic features).

Of the remaining groups, the pollinaria of Oxypetalinae are generally large, and structurally complex, sometimes with teeth on the flattened translator arms. Flowers also tend to be large, and generally the stylar head extends beyond the staminal column as a well-developed appendage.

Genera of the Metastelmatinae and remaining unplaced groupings tend to have minute or small flowers.

General Description

Status

All native except those highlighted in list above [Stapelia, Calotropis, Gomphocarpus]. Most endemic to the New World, but Marsdenia is pan-tropical, and Asclepias is most diverse in North America and Africa, with just 12 species extending into central and South America.

Notes on delimitation

The Asclepiadoideae is the most derived of five subfamilies now included within an expanded Apocynaceae.

Its extreme floral modifications, with complicated pollination syndromes and structures, often with corona elements in one or more whorls, and with pollen aggregated into pollinaria, led earlier workers to classify it as a separate family; but in reality the trend towards the complicated flowers of the Asclepiadoideae starts midway through Apocynaceae s.s., where the stylar head abutts closely underneath the cone of anthers, forming a functional gynostegium. From there on, progressive synorganisation of floral organs, accompanied by a shortening of the corolla tube, lead to greater control over pollen transfer, through the Apocynoideae and the Old-World subfamilies Periplocoideae and Secamonoideae, to the highly derived Asclepiadoideae.

The basal subfamily Rauvolfioideae is much more variable both vegetatively and florally than these more derived groups.

Number of genera

c. 50 genera native to neotropics (largest genera listed first in each group, then alphabetical)

Marsdenieae

Marsdenia R.Br.

Ceropegieae

[Stapelia L., and possibly other genera - introduced]

Asclepiadeae: Asclepiadinae

Asclepias L.

[Calotropis R.Br., Gomphocarpus R.Br. - introduced]

Asclepiadeae: Cynanchineae

Cynanchum L. [note only c. 12 New World species belong here - most NW 'Cynanchum' are now referable to genera in the Metastelmatinae or Orthosiinae]

Goyder, D. J. (2004). An amplified concept of Philibertia Kunth (Apocynaceae: Asclepiadoideae), with a synopsis of the genus. Kew Bull. 59: 415 - 451. [Includes key to genera of Oxypetalum group, and key to species of Philibertia]

Inflorescence cymose, most frequently umbelliform but sometimes with flowers more or less racemosely fasciculate along a simple or branched rhachis

Flowers

Flowers regular, pentamerous; calyx-tube very short or obsolete; corolla contorted, imbricate or valvate, gamopetalous, the lobes sometimes connivent at the apex; filaments united into a tube, anthers connivent throughout their length and united with the expanded style apex, introrse, bilocular and provided with lateral horny wings

Corona

Corona arising from the staminal column, very variable in form or rarely absent or reduced to five small fleshy tubercles

Androecium

Pollen in waxy masses (pollinia) attached in pairs by caudicles of various form to 5 corpuscular usually horny sutured pollen carriers arising from the style apex and concealing the stigmatic surfaces

Gynoecium

Ovary of two separate carpels; styles free to the apex where they are united in a peltate disk which is convex, conical or beaked; ovules multiseriate, on a single adaxial placenta in each carpel

Fruits

Fruit of two (by abortion often one) erect or divergent follicles which may be linear to ovoid or ellipsoidal, membranous to woody, smooth or winged or variously armed with soft or indurated prickles, always dehiscent lengthwise adaxially

Seeds

Seeds compressed, often with a membranous or thickened margin and nearly always crowned with a coma of silky hairs

Climbing, twining or erect, softly woody or occasionally with wiry stems, or herbaceous perennials with a tuberous rootstock; leaves opposite, entire, linear to very broad, pinnately nerved; stipules absent, but sometimes with a stipular frill around the stem at the nodes which may become woody; inflorescence a terminal cyme often lateral by sympodial growth of the axis, sometimes racemiform or umbelliform; bracts very small; flowers bisexual, sometimes functionally unisexual and then dioecious; calyx-tube very short or obsolescent, segments 5, valvate or imbricate but opening very early; corolla gamopetalous, 5-lobed, tube short or sometimes as long as or longer than the lobes, lobes contorted or very rarely valvate; corona of 5 lobes arising from the base of the stamen filaments, of varied form, or rarely absent or reduced to tubercles; stamens 5, inserted in the corolla-tube or at its throat, free from each other, but with the anthers pressed together and to the expanded apex of the style; anthers basifixed, introrse, usually with a scarious apical appendage inflexed over the style; pollen granular, in tetrads discharged on to 5 spathulate pollen carriers derived from the expanded part of the style and with a glandular base; pollen carriers concealing the 5 stigmatic surfaces; carpels two, joined only by their styles, which unite to form a flat or variously elongated head; ovules numerous, multiseriate on a single adaxial placenta; fruit of two (or one by abortion or pollination failure) divergent or reflexed sessile follicles; follicles more or less fusiform and often greatly elongated, or sometimes broadly ovoid, smooth, glabrous or hairy, sometimes warted or winged, finally dehiscing lengthwise adaxially; seeds flat, often winged, and crowned with a coma of soft silky hairs; endosperm present; embryo straight, almost as long as the seed, cotyledons flat